On Wed, Mar 18, 2020 at 10:36:10AM -0700, Dan Williams wrote: > On Wed, Mar 18, 2020 at 1:20 AM George Spelvin <lkml@xxxxxxx> wrote: >> On Tue, Mar 17, 2020 at 08:53:55PM -0700, Dan Williams wrote: >>> I had the impression that unless unlikely is "mostly never" then it >>> can do more harm than good. Is a branch guaranteed to be taken every >>> BITS_PER_LONG'th occurrence really a candidate for unlikely() >>> annotation? >> >> I had to look this up. GCC manual: >> >> For the purposes of branch prediction optimizations, the probability >> that a '__builtin_expect' expression is 'true' is controlled by GCC's >> 'builtin-expect-probability' parameter, which defaults to 90%. You can >> also use '__builtin_expect_with_probability' to explicitly assign a >> probability value to individual expressions. >> >> So I think that <= 10% is good enough, which is true in this case. >> >> I was tring to encourage the compiler to: >> * Place this code path out of line, and >> * Not do the stack manipulations (build a frame, spill registers) >> needed for a non-leaf function if this path isn't taken. > > Understood, I think it's ok in this case because the shuffling only > happens for order-10 page free events by default so it will be > difficult to measure the perf impact either way. But in other kernel > contexts I think unlikely() annotation should come with numbers, 90% > not taken is not sufficient in and of itself. I'm not sure I fully understand your point. I *think* you're editorializing on unlikely() in general and not this specific code, but it's a little hard to follow. Your mention of "order-10 page free events" is confusing. Do you mean "(order-10 page) free events", i.e. freeing of 1024 consecutive pages? Or are you using "order" as a synonym for "approximately" and you mean "approximately 10 (page free event)s"? We both agree (I hope) that the number here is obvious on brief inspection: 1/BITS_PER_LONG. GCC's heuristics are tuned to value cycles on the fast path 9x as much as cycles on the slow path, so it will spend up to 9 cycles on the slow path to save a cycle on the fast path. I've found one comment (https://pastebin.com/S8Y8tqZy) saying that GCC < 9.x was a lot sloppier on the cost ratio and could pessimize the code if the branch was more than ~ 1% taken. Perhaps that's what you're remembering? Fortunately, 1/64 = 1.56% is fairly close to 1%. so I'm not too worried. > You can add: > > Acked-by: Dan Williams <dan.j.williams@xxxxxxxxx> Thank you!
